The power consumption associated with any complex electronic system or device, such as a VLSI (Very Large Scale Integration) system, a computer system, or a processor, can be quite large and costly. The large power consumption of high-performance computer systems, for instance, make the design of power distribution and cooling sub-systems very challenging and often very costly. A processor of such a high-performance computer system typically will dissipate a large component of the total power of the computer system and therefore the power dissipation characteristics of the processor are customarily optimized in order to reduce the overall cost of the computer system. Optimizing the power characteristics of the processor in order to minimize power dissipation focuses on the goal of reducing maximum power, reducing average power and reducing step load of the computer system. The relative advantages and disadvantages of these design goals will now be explored in turn.
Reducing Maximum Power
Reducing the maximum power provides several benefits to the cooling sub-system and the power distribution sub-system of an electronic system or device. In terms of the cooling sub-system of a computer system, for example, when a processor of the computer system dissipates maximum power it also generates maximum heat. In order to prevent overheating of the computer system, the cooling sub-system must, ideally, remove heat as quickly as it is generated. High-power processors often require quite aggressive and expensive cooling solutions to adequately remove generated heat.
With respect to the power distribution sub-system of an electronic system, the power supply of the system must deliver the maximum current demanded by the system. In a computer system in which the processor requires a large portion of available system power, maximum power of the processor can be expected to significantly increase the power rating required of the system power supply. Power supplies possessing the requisite power rating for a computer system are expensive, which again causes overall system cost to increase. Additionally, a high-power component of the system, such as a processor, may increase the power requirements of the system such that a single standard power cord is no longer capable of supplying the required power. A room in which an electronic system or device having special power requirements is to be used may have to be rewired to accommodate non-standard power supplies, for instance.
In addition to increasing system cost, the maximum power of an electronic system or device, such as the processor of a computer system, may also have an adverse impact on system performance. The power dissipated by a processor, for instance, is translated into heat that slows down the operation of the processor as well as other system components. Therefore, reducing the maximum power of the processor reduces heat generation. Reduced heat generation translates into higher processor operating frequencies that will in turn increase overall system performance. CMOS (complementary metal oxide semiconductor) circuits, in particular, are often used in processors and are known to operate faster at lower temperatures.
Multi-processor systems present special power concerns. In multi-processor systems, performance is a function of the number of processors used in parallel in a particular multi-processor system. Because each new processor added to the system increases the total maximum power of the system, the number of processors of the multi-processor system present obvious power supply and cooling considerations. Reducing the maximum power of each processor of the multi-processor system reduces the total maximum power and heat generated by the system. Or, more processors can be used without increasing total maximum power and heat of the system which in turn increases the speed and performance of the system.
Reducing Average Power
Reducing the average power of a device or sub-system of an electronic system means that less power is dissipated over a given period of time. Reducing the average power of a processor, for instance, will reduce the total energy consumed over time by the computer system containing that processor. The ability to reduce average power is especially desirable in portable computers that are capable of running off battery power. In a portable computer, reducing average power enables the portable computer to have a longer battery life for a given battery size, or it allows a less expensive battery to be used to achieve an equivalent battery life.
For non-portable computer systems that do not operate off battery power, reducing the average power of the system provides the advantage of lowering the cost of powering the computer system over time. Thus, reducing the average power of a processor of the non-portable computer system will reduce the cost of providing power to the system over time. While this is a desirable advantage in so far as utility costs will be reduced, such a cost savings is not usually a key factor in the decision of whether to purchase a non-portable computer system, particularly for high-end non-portable computer systems.
Reducing Step Load
For a system to operate correctly, the power supply must be capable of maintaining voltage levels within a tight tolerance. If power consumption of the system varies widely over a short period of time, this greatly complicates power distribution which in turn increases the total cost of the system. Variation of power consumption, called step load, is the difference between maximum and minimum power consumption of the system.
Step load causes should be minimized because the change in current of the step load can be orders of magnitude faster than the reaction time of the power supply. A typical power supply reacts to changing current in a matter of milliseconds while a modern microprocessor is capable of switching from its minimum current to its maximum current in nanoseconds. Until the power supply can react to changes in the current, the power distribution network of the system must have enough capacitance to supply or sink the extra current. If the capacitance of the power distribution network is insufficient, the voltage of the system will droop or surge. Additionally, the power distribution network typically possesses significant inductance properties. While the current is constant, this inductance poses no concern. A sudden change in step load current over time, dl/dt, however, can create large noise events that result in voltage spikes and/or ringing of the system power supply.
As electronic systems and devices have become more complex, step load has correspondingly increased. Increased operating frequencies have also caused the time constant of step loads to decrease. Together, these factors have made the step load phenomenon increasingly important and this trend is likely to continue. In fact, if a current state-of-the-art microprocessor is not designed with step load in mind, the occurrence of step load may become more than a cost issue and may actually make the system containing the microprocessor incapable of being manufactured.
Power-Down Circuitry Techniques
The most common technique used to improve power characteristics of a processor is to power-down unused circuitry or functional areas of the processor. Consider a typical microprocessor capable of executing one of three types of instructions, integer, floating-point, or memory, per cycle. An amount of power is associated with each type of instruction as well with overhead. As an example, assume the following power dissipation of integer, floating-point, and memory functional units as well as with overhead of the processor illustrated in Table 1:
TABLE 1 ______________________________________ overhead 5 W integer 5 W floating-point 10 W memory 5 W ______________________________________
If an unused functional unit of the processor is not powered-down, the maximum power dissipation of the processor will be the sum of the power dissipation of each functional unit type plus overhead, or 25 W. Since, in this example, the overhead, integer, floating-point and memory functional units of the processor are always on, the minimum power will also be 25 W. The difference between the maximum power and the minimum power of the processor, or step load, is thus zero. Finally, average power will also be 25 W. The maximum power, minimum power, average power and step load of such a processor that does not employ power-down circuitry are illustrated in Table 2.
TABLE 2 ______________________________________ Processor without power-down circuitry: ______________________________________ max power = overhead + integer + floating point + memory = 25 W min power = max power = 25 W 25 W .ltoreq. average power .ltoreq. 25 W step load = max power - min power = 0 W ______________________________________
If, however, the type of instruction currently being executed is detected such that unused functional unit or circuitry not currently executing instructions may be powered-down, then the maximum power can be reduced. According to the power dissipation values of this example, the maximum power consumption of the processor, then, occurs when the floating point functional unit of the processor is executing a floating point instruction and thus the integer and memory portions of the processor are inactive. The maximum power consumption in this instance is 15 W and is determined only by the floating-point circuitry, which consumes 10 W, and the overhead circuitry of the processor, which consumes 5 W. The minimum power consumption of 5 W occurs when the processor is idle and only overhead power is consumed. In this example, the difference between the maximum power consumption and the minimum power consumption, or step load, is 10 W. Average power will be between 5 W and 15 W depending on the instruction mix (integer, floating-point and memory) of the processor. Table 3 illustrates the maximum power, minimum power, average power and step load of a processor that does use power-down circuitry.
TABLE 3 ______________________________________ Processor with power-down circuitry: ______________________________________ max power = overhead + Max (integer, floating point, or memory) = 25 W min power = overhead = 5 W 5 W .ltoreq. average power .ltoreq. 15 W step load = max power - min power = 10 W ______________________________________
Table 3 illustrates that the use of power-down circuitry successfully reduces maximum and average power at a cost of increasing step load. In the example used, maximum power was reduced from 25 W to 15 W, average power was reduced from 25 W to a range of between 5 W and 15 W, and step load increased from 0 W to 10 W. For some systems, particularly portable computers, power-down circuitry likely provides the best trade-off between step load and power parameters. For non-portable computer systems, however, the increase in step load may significantly increase system cost, making the system effectively incapable of being manufactured. There is therefore an unmet need in the art to be able to reduce the maximum power of an electronic system, sub-system or device without increasing the step load of the electronic system, sub-system or device.
It is therefore an object of the present invention to reduce maximum power of an electronic system, sub-system or device.
It is further an object of the present invention to reduce maximum power of an electronic system, sub-system or device without increasing step load of the electronic system, sub-system or device.
It is still a further object of the present invention to reduce maximum power of an electronic system, sub-system or device while simultaneously reducing the step load of the electronic system, sub-system or device.
It is yet another object of the invention to reduce the step load of an electronic system, sub-system or device beyond that achievable using current power-down circuitry techniques while maintaining a reduction in maximum power achievable with power-down circuitry techniques.